THE ELBOW, FOREARM, AND CARPUS
The elbow joint projects on the ventral ends of the fourth and fifth ribs. The olecranon, the medial and lateral epicondyles of the humerus, and the robust collateral ligaments are all easily palpable and provide the necessary orientation for joint puncture.
This is performed from the lateral aspect with the needle directed between the lateral epicondyle and the olecranon to enter a considerable pouch of the capsule within the deep olecranon fossa.The ulna is complete but slender, and it is the massive radius that bears the weight. As always the subcutaneous medial border of the radius marks the division between the cranial extensor and caudal flexor muscle groups (Figure 30-3). The ulna is palpable only at its extremities, the olecranon and lateral styloid process. In most subjects the forearm inclines mediodistally to the
Figure 30-1 “Wing shoulder” in a 6-year-old Jersey cow.
carpus while the foot angles laterally, producing a “knock-kneed” stance. Although straight limbs are preferred, this inward bulging of the carpus does not appear to be a disadvantage.
The proximal row of the carpal skeleton comprises radial, intermediate, and ulnar carpal bones. The upper and lower borders of the accessory bone provide rough guides to the levels of the antebrachiocarpal and mid- carpal joints. The distal row consists of only two bones: fused second and third carpals and the fourth carpal (see Figure 2-48). In theory, movement is possible at all three levels but most occurs between the forearm and carpus; a moderate amount takes place at the middle joint, and next to none occurs at the carpometacarpal level. Movements other than flexion and extension are largely prevented by the many ligaments, of which the collateral pair is most important.
The cavities of the two distal joints always communicate; occasionally all three do so. Puncture is possible at the proximal and middle levels and is obviously most easily performed when the joint is flexed.Irregularities of the palmar aspect of the carpal bones are covered and smoothed by the thick fibrous layer of the joint capsule (palmar carpal ligament), which combines with the accessory bone and flexor retinaculum to enclose the carpal canal. The joint capsule also bends dorsally with deep fascia to form the extensor retinaculum that binds the extensor tendons in place. An inconstant bursa between this retinaculum and the skin occasionally enlarges to form an unsightly but painless blemish (hygroma).
Only the digital extensors and flexors among the muscles of the forearm merit notice. The common digital extensor has two bellies: the larger medial one extends its tendon of insertion to the medial digit, and the smaller lateral belly has a tendon that splits at the fetlock to insert on both digits. The two tendons share a synovial sheath where they descend over the carpus. The lateral digital extensor comports itself like the medial belly of the common extensor (Figure 30-2). The superficial digital flexor also possesses two bellies. The tendon of the deep one passes through the carpal
Figure 30-2 Muscles of the bovine forelimb, lateral view. 1, 1', Trapezius; 2, supraspinatus; 3, deltoideus; 4, latissimus dorsi; 5, brachiocephalicus; 6, biceps; 7, 7’, long and lateral heads of triceps; 8, brachialis; 9, extensor carpi radialis; 10, common digital extensor; 10', tendon of lateral belly; 11, 11', lateral digital extensor and its tendon; 12, extensor carpi obliquus; 13, ulnar head of deep digital flexor; 14, ulnaris lateralis.
canal, and that of the superficial one remains outside the flexor retinaculum; both are protected by long synovial sheaths that extend beyond the carpus into the cannon where the tendons merge. The three bellies of the deep flexor give rise to a stout common tendon that passes through the carpal canal, where it also receives synovial protection. All these tendons receive fuller notice later.